Vehicle powertrains may include transmissions to transfer power and torque from an engine to the vehicle output (axles or wheels). Hybrid powertrains may include multiple primary movers, including internal combustion engines or alternative power sources, such as one or more electric machines coupled with an energy storage device. When the powertrain is equipped with an additional primary mover, such as an electric machine, the transmission may also transfer torque and power from the electric machine to provide traction for the vehicle. Fixed gears or speed ratios of the transmission allow the engine to operate within a narrow range of speeds while providing a wide range of output speeds to the powertrain.
Different gears or operating modes of the transmission provide different, selectable speed ratios or gear ratios, and also different levels of torque multiplication. Changing the speed ratio of the transmission generally changes the ratio of input speed to the output speed and changes the torque multiplication between the input and the output. Selection of speed ratios may occur through actuation of hydraulic components controlled by one or more hydraulic valves. The hydraulic pressure and flow regulated or provided by these valves selects the requested speed ratio.